Coral reefs are one of the most fragile, biologically complex, and diverse marine ecosystem on Earth. This ecosystem is one of the fascinating paradoxes of the bio- sphere: how do clear, and thus nutrient-poor, waters sup- (5) port such prolific and productive communities? Part of the answer lies within the tissues of the corals themselves. Symbiotic cells of algae known as zooxanthellae carry out photosynthesis using the metabolic wastes of the coral thereby producing food for themselves, for their corals, (10) hosts, and even for other members of the reef community. This symbiotic process allows organisms in the reef com- munity to use sparse nutrient resources efficiently. Unfortunately for coral reefs, however, a variety of human activities are causing worldwide degradation of (15) shallow marine habitats by adding nutrients to the (water. Agriculture, slash-and-burn land clearing, sewage disposal and manufacturing that creates waste by-products all increase nutrient loads in these waters. Typical symptoms of reef decline are destabilized herbivore populations and (20)an increasing abundance of algae and filter-feeding animals. Declines in reef communities are consistent with observa- tions that nutrient input is increasing in direct proportion to growing human populations, thereby threatening reef com- (25) munities sensitive to subtle changes in nutrient input to their waters. The passage is primarily concerned with______